This is original patch from Simon Wilkinson with with the following additions and modifications:

 * OpenSSL 1.1.0 support
 * Migrate to new buffer API
 * Support for selection of GSSAPI kex Algorithms
 * The new GSSAPI Key exchange methods with DH and SHA2 (RFC 8732)
 * The new GSSAPI Key exchange using ECDH and SHA2 (RFC 8732)
 * Correctly build without OpenSSL (gss-kex requires openssl)
 * Make sure the kerberos tickets are cleaned up with the user context (#1683295)
 * Port to new packet API in OpenSSH 8.0p1
 * ssh: Allow to query for the GSSAPI Kex algorithms from ssh